1. Field of the Invention
The present invention relates to an inching valve for power-shift transmissions, particularly for lift trucks.
2. Description of Related Art
Inching valves associated with power-shift transmissions (which use several multiple-disc clutches engageable selectively to obtain the various gear ratios) are well known in the art and are used to achieve a slow forward speed of the lift truck, for example, during the stage when it approaches a load to be lifted, even when the internal combustion engine of the truck is kept running at full speed to enable the lifting device to be raised rapidly.
In more conventional solutions, the stem of the inching valve is controlled by the operator by means of a pedal which is connected to the stem through a mechanical transmission. More recent solutions provide for the hydraulic control of the stem of the inching valve. In this case also, the operator controls the valve by means of a pedal but the action of the pedal is transmitted to the valve stem by means of a hydraulic device. With this technique, the piston which controls the stem of the inching valve is operated by the same fluid as is used in the vehicle's braking circuit. In this case, the pedal which controls the inching valve is also the brake pedal of the vehicle.
With inching valves produced in accordance with this known technique, however, there is a risk of the circuit for the brake fluid being contaminated from the hydraulic circuit of the power-shift transmission. Typically, the fluid used in the power-shift transmission, which is also intended to pass through the inching valve, is a lubricating oil which is incompatible with the fluid normally used in the braking circuit. The seals used for the conventional mineral oil which flows through the inching valve are not compatible with the fluid used in the braking circuit, and vice versa. Sometimes, some of the mineral oil from the transmission enters the circuit reserved for the brake fluid and mixes with that fluid, causing the rapid deterioration of the seals of the master cylinder and wheel cylinders of the vehicle's brakes.